Waveguide type displacement interferometer having two reference paths

ABSTRACT

The interferometer comprises a light source, at least one beam splitting means, a beam combiner means, measuring and reference arms as well as wave guides for guiding the light to the beam splitting means and for guiding the light back to photodetectors. Beam splitting means and beam combiner means are formed by integrated optics elements on a substrate. The wave guides on the substrate form a measuring arm (7 1 ) and two reference arms (6 1 , 8 1 ) which have different optical path lengths for setting a phase difference. An evaluation electronics for determining correct as to signs, the change in optical path length in the measuring arm (7 1 ) is connected to the photodetectors.

The invention relates to an interferometer according to the preamble ofthe claim as well as a method for determining correct as to signs, thechange in the optical path length with the aid of said interferometer.

A Michelson interferometer which is built in the manner of integratedoptics is described in German disclosure letter 38 25 606. Theinterferometer consists of a monochromatic laser light source, a beamsplitting means, a reference mirror and a measuring mirror as well aslight guide fibers for guiding the light to the beam splitting means andfor guiding the light carrying the measurement signal back to adetector. Therein, the beam splitting means is formed by anintegrated-optical, symmetrical coupling capacitor in a glass substratein which wave guides produced by struturated ion exchange are arranged,which on one hand form the measuring arm and on the other hand thereference arm of the interferometer.

The invention, therefore, is based on the object of giving aninterferometer of the kind cited in the beginning, such a design that itis capable of proving optical path lengths changes with high resolutionand determined as to direction in a simple and reliable manner.

The invention solves said object with the aid of the features of thecharacterizing clause of claim 1. Advantageous embodiments of theinvention are subject of the subclaims. Furthermore, a method is statedwhich, with the aid of the interferometer according to the presentinvention, renders possible the measurement of optical path lengthschanges determined as to direction and with great accuracy.

With the aid of the interferometer according to the present inventionand the method according to the present invention, respectively, it is,thus, possible to carry out high-resolution position measurements aswell as to determine changes in the refractive index. By the combinationof counting methods and single-sideband modulation, therefore, alsolarge path distance changes can be quickly displayed in a mannerdetermined as to directions (counting method) and the exact value of theoptical path distance change or the change in refractive index,respectively, can be recognized (single-sideband method).

Embodiments of the invention now are described with reference to theattached drawings. Therein:

FIG. 1 shows the schematical construction of an integrated opticsMichelson interferometer;

FIG. 2 shows the switching plan of evaluation electronics;

FIG. 3 shows a variation of a Michelson interferometer with directillumination;

FIG. 4 shows a variation of a Michelson interferometer with passivephase modulator;

FIG. 5 shows a Michelson interferometer with Bragg reflectors;

FIG. 6 shows an interferometor as gas sensor;

FIG. 7 shows a Michelson interferometer with decoupling gate in sideview;

FIG. 8 shows a Michelson interferometer with decoupling gate in topview;

FIG. 9 shows a refractometer in side view;

FIG. 10 shows a refractometer in top view;

FIG. 11 shows a double Mach-Zehnder interferomter as sensor;

FIG. 12 shows a double Mach-Zehnder interferometer for distancemeasurement in side view;

FIG. 13 shows a double Mach-Zehnder interferometer for distancemeasurement in top view.

In the following at first quite in general, an embodiment of a Michelsoninterferometer according to the present invention is described. In theinterferometer, serveral beam splitting means are provided for in theglass substrate so that one common arm as measuring arm and two arms asreference arms will result. In the common arm for example the light of ahelium-neon laser is coupled in through a wave guide, preferably a glassfiber. Subsequent to the coupling-in point the measuring arm separatesinto two wave guide structures running in the same direction as thereference arms and which are guided to meet again in the further course.Thus, two Y-shaped branchings are formed, in which the light is firstseparated and then combined, respectively. Thus, two direction couplers(3 dB) are formed which serve as beam splitting means. The reflectedlight from the reference mirrors which, for example, consist of aluminumdeposited on the wave guide end faces, interferes with the lightreflected from the object mirror. The interference signals are suppliedto two detectors via multiple-mode fibers coupled to the front face ofthe substrate. A thermo-optical modulator on one of the two referencearms pemits the setting of 90° phase difference between the tworeference arms, whereby a quick forward/backward direction recognitionis effected. For the purpose of setting of a 90° phase difference,several methods are at offer. One method consists in that thethermo-optical modulator is driven by direct voltage. In a passivemethod, the properties of the wave guide are affected by a defineddeposition of layers, so-called over-layers. This is done by depositionof suitable substances, like adhesives for example or by evaporation orsputtering of silicon oxide layers for example. The desired phasedisplacement is thus set already during the manufacture of the chips. Inanother passive method, a thin layer is removed above the wave guide inorder to affect the properties of the wave guide. Furthermore, theproperties of the wave guide may be influenced by dimensioning such asthe width or density of the waveguide.

The thus manipulated signals are supplied by the detectors to theevaluation electronics described later. Therein, two evaluationprocesses are carried out in parallel. The "strip counting" withorthogonal detector signals is combined with a phase measurement byoptical superposition reception. Changes in path length with aresolution corresponding to a fraction of the wavelength (λ/n) anddirection detection are found with the quadrature signal and thecounting method. High resolution phase detection (e.g. λ/100) is carriedout using single-sideband modulation.

For being able to detect quick movements of the object mirror, the peakvalues of a detector signal are counted. This counting mode correspondsto counting rings in a classical interferometer. In case of such aclassical arrangement, the direction of movement is recognizable fromthe converging and diverging of the rings and/or from the signal curve,of two detectors displaced by 1/2 ring breadth. In the case ofintergrated optics arrangement only single-mode wave guides are used andthere is only one intensity modulation. In the abovedescribed passivemeasuring method, direction detection is obtained by the one ofreference arms being shifted in phase by 90° with respect to the otherreference arm. A detector signal provides the counting pulses for thecounter, the second detector signal is orthogonal with respect theretoand determines the counting direction resulting from the direction ofmovement of the measuring mirror.

The high-resolution evaluation of the mirror position is effected bymeans of single-sideband modulation through the use of an thermo-opticalmodulator in the measuring arm of the interferometer. By the opticalinterference method, the optical phase rotation being proportional tothe change in distance, is converted into a low-frequency electricalphase rotation. The phase found using a commercial phasemeter can bemeasured to about 1°. This corresponds to a resolution in the change indistance of about 1 nm, depending on the wavelength of the light source.

Now, a more detailed account is made of the counting process.

A quick forward/backward recognition (direction discrimination) andmeasuring value detection is achieved by the counting method. We starton the assumption that the detector signals are mutually shifted inphase by 90°. On the integrated optics chip two detector signals aredirectly generated. For bringing their power characteristic into a phasedifference of 90° with respect to one another, a thermo-opticalmodulator is operated for example with direct power. The phasedifference is adjusted with an oscilloscope. The detector signals thusset orthogonally, serve for forward/backward recognition or directiondiscrimination, respectively. The direction signal is passed to thecounter as TTL signal. The signal of a detector is delivered to thecounter in form of spike pulses. Without single-sideband modulation thisarrangement is sufficient. If the phase shifter in the upper referencearm is replaced by the above-stated passive methods such as adeposition, the interferometer sensor is to be operated--as has alreadybeen mentioned--completely without electrical connection.

For the higher resolution, phase shifter in the measuring arm iscontrolled by a function generator and amplifier. In this case themodulation is carried out with a frequency of 1 kHz. As the phaseshifter operates on the measuring arm of the interferometer, the phasemodulation acts on both reference arms equally. The counter follows theone (1) kHz modulation without problems. Therein, continuous unitscorresponding to fractions of the wavelength (λ/n) are counted in upwardand downward directions in correspondance to the 2π modulation for thesingle-sideband method. Thus the last digits of the display are jumpingand cannot be read by the eye. For the purpose of measuring valuerepresentation on the digital display with e.g. λ/n, for example aresolution of the lower digits are written into a memory under triggercontrol of the function generator. Thus, the display is coupled to adefinite phase of the dynamic phase shifter in the measuring arm.

In the evaluation unit the direction of movement of the measuring mirroris additionally displayed with light emitting diodes which are connectedto the forward/backward recognition. This signal permits to effectoscillation peak measurements in simple manner in that it is put on thegate of a counter.

In the single-sideband method the phase-modulated single-sidebanddetection linearly transfers peak and phase of an optical signal into alow-frequency peak and phase which are measured separately from oneanother. In case of a sufficient single-sideband modulation there is astrict frequency shift. A modulation is impressed onto the opticalcarrier such that one sideband just disappears. Thus it is possible toconvert the optical phase into a electrical phase measurement moreeasily.

As the phase shift of a themo-optical modulator is not linearlycorrelated to the control signal, the possibility of an electrical phasereadjustment in the measuring arm of the interferometer is not realized.The 2π disconuities, too, have a disadvantageous effect on thecontroller. As, here, it is the matter of power-controlledthermo-optical modulators, a phase following action is too sluggish andthe thermal load imposed in the chip would depend on the measuringphase. For the above reasons, an evaluation of the relation in phasebetween measuring arm and reference arm is designed such that thesingle-sideband modulator continuously runs with the same selectionfunction, that it does not disturb the counting mode of theinterferometer and due to the electric output constant to eachmodulation period, leaves the interferometer in thermal equilibrance.The phasemeter compares the phase of the light in the measuring arm withthe one of the reference arm and displays it in digital manner.

For the measuring accuracy endeavoured, the single-sideband methodrequires an accurate setting of the phase swing. In thermo-optical phasemodulators the modulator characteristic has to be included into thecalculation of the selection function.

In single-sideband suppression, there are analogue and digitalmodulation functions. In analogue manner the single-sideband modulationcan be effected by a saw-tooth like control of the modulator. Onesideband can thereby be suppressed completely. The relating frequency isfiltered out by means of a band-pass filter connected subsequently tothe detector. If the single-sideband suppression is not complete, thiswill reduce the accuracy of phase measurement. In this analogue method,the peak phase swing has to be met with high exactness for observing asufficiently high single-sideband suppression and thus achieving a highaccuracy.

This saw-tooth like modulation function, however, can be replaced by astepped digital modulation function with a 2π phase rotation within oneperiod. Digital control funtions provide the advantage of beingrealizeable with processor controls.

Furthermore, there are digital modulation functions which run in ameander form around a saw-tooth like curve and thus tolerate deviationsof the peak phase swing from the optimum value. This function is clearlyless sensible with respect to deviations from the optimum functioncurve.

In the evaluation electronics the modulation voltages of the individualsteps are adjustable, which then are switched to the phase modulator inalternating manner. The repetition frequency therein amounts to 1 kHz. Afree adjustability of the selection steps for the thermo-opticalmodulators is carried out either by a potentiometer adjustment for eachstep and the respective phase swing or by a processor with D/A converterand software-controlled step function. Thereby account is taken for thebehavior of the thermo-optical modulator. On one hand the optical phaserotation is proportional to the supplied electrical power and thusproportional to the square of the applied voltage, on the other hand theresistance of the heating electrode can be independant from temperature.With adjustable steps, suitably predistorted signals of the functiongenerator may be made use of.

In the following the embodiments shown in the drawings are described indetail.

The embodiment of the interferometer under FIG. 1 comprises a glasssubstrate 1. Therein, three light guide fibers 3, 4, and 5 are coupled,e.g. glued, to the front face 2 of the substrate 1, wherein the lightguide fiber 4 serves for the light originating from a laser (not shown)and the light guide fibers 3 and 5 lead to one photodetector (not shown)each. In the substrate 1 the light is guided in wave guides 6, 7 and 8which have been prepared by a field-supported exchange of potassiumions. The ion exchange is carried out for example with a supportingelectrical field of 50 V/mm and at a temperature of 450° C. The ionexchange using typical exchange periods of 10 sec is charge-controlledin order to guarantee a reproducable wave guide generation. Thestrip-type wave guides are dimensioned for a single-wave operation atthe wavelength λ=0.633 μm. The wave guide 7 split into two arms at 9,which arms approach the wave guides 6 and 8 at 10 and 11 and are ledtogether again at 12. The measuring arm 7₁ of the wave guide 7 ends atthe front face 13 of the substrate 1 in front of a rate-of-change indexlens 14 which is directly glued to the end face 13 of the substrate 1the measuring arm 7₁ for colliminating the light. An object mirror 15 isdisposed at a distance from lens. The wave guides 6 and 8 approach waveguide 7 at 10 and 11. The reference arms 6₁ and 8₁ of the wave guides 6and 8 end at the end face 13 of the substrate 1 in front of referencemirrors 16 and 17 deposited on the end face 13 of the substrate 1. Thelight coming from the laser and coupled into wave guide 7 by the lightguide fiber 4 is split in the two direction couplers 10 and 11. Thereflected light from the reference mirrors 16 and 17 interferes with thelight reflected from the object mirror 15 at 10 and 11. The interferencesignals are supplied to the two photodectectors through themultiple-mode fibers 3 and 5 coupled to the front face to the substrate.A thermo-optical modulator 18 on the reference arm 6₁ permits thesetting of 90° phase difference between the two reference arms 6₁ and8₁, whereby a quick forward/backward recognition can be effected. At thelevel of the thermo-optical modulator 18 another thermo-opticalmodulator 19 for high-resolution measurements by means of phasemodulation and single-sideband detection is disposed on the measuringarm 7₁. The heating electrodes of the thermo-optical modulators 18 and19 consist of an evaporated aluminum layer of 400 nm thickness. Theelectrodes are structured in a photolitographical way with a conductorwidth of 4 μm, a length of 7 mm and a mutual distance of 6 μm.

The signals supplied by the light guide fibers 3 and 5 to thephotodetectors 20 and 21 are further processed with the aid of anevaluation electronics schematically shown in FIG. 2. Both detectorsignals are pre-amplified in the amplifiers 22 ad 23 and are supplied toa pulse former 24, 25 (Schmitt trigger) in which the arriving signalsare converted into rectangular pulses. In a direction discriminator 27the signal is converted into spike pulses by one of the two pulsegenerators 24, 25 and supplied to a counter 28. By comparing the signalsof the two pulse generators 24, 25, the direction discriminator 27decides whether the displacement of the mirror is a forward or backwardmotion, and issues a signal V/R to the counter 28.

For the higher resolution, the thermo-optical modulator 19 is selectedthrough a function generator 29 and a subsequent amplifier 30. As themodulator 19 is located on the measuring arm 7₁ of the interferometer,the phase modulation acts on both reference arms 6₁, 8₁ equally. Themodulation in this case is carried out at a frequency of 1 kHz which caneasiliy be followed by the counter 28. Therein it is continuouslycounted in upward and downward directions in λ/n units corresponding tothe 2π phase modulation for the single-sideband method. For measuringvalue representation on a digital display 31 with λ/4 resolution, thetwo lower digits are written into a memory 32 in triggered form by thefunction generator 29. Thus, the display is coupled to a defined phaseof the dynamical phase shifter, i.e. the thermo-optical modulator 19.The frequency shift in the measuring arm 7₁ is effected with the aid ofthe thermo-optical modulator 19 by means of single-sideband modulation.The modulation is carried out in such a manner that in the spectrumproduced around the carrier frequency one of a pair of sidebandsdisappears. Therein, there exist analogue and digital modulationfunctions for the single-sideband suppression. In analogue manner thesingle-sideband modulation can be carried out by a saw-tooth likeselection of the modulator 19. One sideband can thereby be suppressedcompletely. The relating frequency is filtered out by a band-pass filter33 connected subsequently to the detector 21.

The saw-tooth like modulation function can, however, also be replaced bya stepped digital modulation function with a 2π phase rotation withinone period.

In the evaluation electronics, the modulation voltages of the individualsteps are adjustable, which then are switched to the phase modulator 19in an alternating manner. The repetition frequency therein is 1 kZh. Aphasemeter 34 which also is connected to the function generator 29,compares the phase of the light in the measuring arm 7₁ to that of thereference arm 8₁ and displays it in digital form in the display unit 35.

The control 36 shown in FIG. 2 in case of single-sideband method servesfor producing accurate rectangular pulses by readjusting the pulsegenerators and amplifiers, respectively.

The display 31 thus shows the change in the optical path determined asto directions with λ/n accuracy, while the display 35 adds the exactintermediate value. The two displays 31 and 35 can be combined into onesingle display by means of a microprocessor.

A further embodiment of the invention results when a window is formed onthe measuring arm 7₁, which window carries a layer being chemicallyaffectable. This enables the direct proof of certain gases. Sucharrangement, however, is already known for a Mach-Zehnderinterferometer, but the apparatus provides the disadvantage that itcannot be seen whether the refractive index increases or decreases. Withthe aid of the described construction of the interferometer, however, itis possible to exactly determine this item, too.

FIG. 3 shows a variation of the interferometer according to the presentinvention. In contrast to the embodiment under FIG. 1 a light source 50and photodetectors 21, 22 are disposed directly at the front face 2 ofthe substrate 1.

In the embodiment of FIG. 4 the adjustment of the phase differencebetween the two reference arms 6₁, 8₁ is carried out in a passive mannerby a transparent layer 51 applied to the reference arm 6₁. For thispurpose, for example, evaporated or sputtered organic layers, like SiO₂,MgF₂ may be used as well as polymers. The same effect is obtained byremoving a defined layer 51 above the reference arm 6₁.

FIG. 5 shows a further embodiment of a Michelson interferometer in whichso-called Bragg grids 52, 53, like those which also are used in laserdiodes for example are applied to the reference arms 61, 81 instead ofthe mirrors 16, 17 at the reference arms 6₁, 8₁ as shown in FIG. 1. Thedesired phase difference between the reference arms 6₁, 8₁ herein is setby means of the different positions of the Bragg grids 52, 53 and thusby means of the different lengths of the reference arms 6₁, 8₁.

A different optical length of the two reference arms 6₁, 8₁ can--as isschematically shown in FIG. 6--also be achieved by a diffent breadth ofthe reference arms 6₁, 8₁. In the example of FIG. 6, the reference arm6₁ is broader than the reference arm 8₁. As the effective refractoryindex valid for the propagation of light in the wave guide depends onthe breadth of the wave guides, both reference arms 6₁, 8₁ havedifferent optical length in spite of their identical geometricallengths. FIG. 6 furthermore shows the use of an embodiment of theinterferometer according to the present invention, of the Michelson typeas sensor for example for gas. The geometrical length of the measuringarm 7₁ here remains constant. The reflector used in the above-describedembodiment, at the end of the measuring arm 7₁ is replaced by astationary Bragg grid 55 on the measuring arm 7₁. A sensor layer 54deposited on the measuring arm 7₁ changes its refractory index independance on the concentration of a gas to be measured or a liquid tobe measured and thus causes a change in the optical path distance in themeasuring arm 7₁, which change is detected.

FIGS. 7 and 8 show a further embodiment for an interferometer of theMichelson type. The laser light source 50 is coupled in through agradient index lense 56 (so-called GRIN lens) into the wave guide 7. Thephotodetectors 20, 21 in this case are not glued to the front face 2 ofthe substrate but are applied to the surface of the wave guides 6, 8.The decoupling of the light into the measuring path bordered by thesubstrate 1 and the reflector 15 in this case is not effected through afront-face mounted collimator electronics 14, but with the aid of aplanar, focussing grid 57. The grid 57, which for example is made ofTiO₂, forms the light leaving the measuring arm 7₁ into a collimatedlight bundle which again is coupled back into the measuring arm 7₁ afterhaving been reflected at the measuring mirror 15 through the same grid57.

FIGS. 9 and 10 show a utilization of an embodiment of the interferometerof the present invention as refractometer by means of which therefractory index of the ambient air can be measured. The geometricallength of the measuring path consisting of the measuring arm 7₁, thecollimator lens 14, an air distance and a fixed end mirror 58, remainsconstant. The end mirror 58 extends over the ends of the reflector arms6₁ and 8₁. A change in the refractory index of the air causes a changein the optical path length in the measuring arm 7₁ without influencingthe reference arms 6₁, 8₁. Using the measured change in optical pathlength, the change in the refractory index of the air can be deteminedwithout ambiguitiy.

FIG. 11 shows an embodiment of the interferometer of the presentinvention as Mach-Zehnder interferometer with two reference arms 6₁, 8₁.In a beam splitting means 59 the light is distributed to the measuringarm 7₁ and the two reference arms 6₁, 8₁. Here, too, the phasedifference between the two reference arms 6₁, 8₁ again is set with theaid of an active (e.g. thermo-optical) or passive (transparent layer)phase modulator 18. The value to be measured, like for example gasconcentration in case of use as gas sensor, influences via the sensorlayer 54 the optical path length of the central measuring arm 7₁. Thelight of the measuring arm 7₁ is split by a beam splitting means 60 andinterferes with one direction couplers 61, 62 with the light of thecorresponding reference arms 6₁, 8₁. Each direction coupler 61, 62 hastwo outputs the light of which is supplied to photodetectors 63, 64, 65,66. In this way counterclock signals are obtained which canadvantageously be used (by subtraction) for suppressing a fraction ofequidirected light. However, it is also possible to use one simple beamcombiner each for the measuring arm 7₁ and the reference arms 6₁, 8₁ inconnection with only one photodetector instead of said two directionalcouplers 61, 62.

FIGS. 12 and 13 show a double Mach-Zehnder interferometer similar tothat of FIG. 11, which however in this case serves for path measurement.A first coupling grid 57 forms the light of the measuring arm 7₁ into acollimated beam bundle which is reflected by a measuring reflector 15 inlaterallly displaced manner and again is coupled back into the measuringarm 7₁ through a second focussing grid 67. In this way the position ofthe measuring reflector 15 can be determined.

In multiple aspects the invention is not limited to the specialtechnical means described in connection with the embodiments. Thus, itis not absolutely necessary to use a laser light source as light sourcefor the interferometer. It is generally known that there is a relationbetween the path length to be measured by the interferometer and thecoherence length of the light of the light source used. In many cases ofuse, therefore, the light of so-called super luminescent diodes, forexample, provides a sufficiently long coherence length capable ofcarrying out the measurement. Depending on the case of use, therefore,also other light sources having light with a sufficient coherence lengthcan be used. For the substrate and the wave guides formed thereto, alsoother materials can be used than glass with wave guides produced by ionexchange. For example, lithium niobate may be used as substrate, whereinthe wave guides in these crystals have been produced by titaniumdiffusion or proton exchange, III-V semiconductors may be used assubstrate, in which the wave guides including so-called Rib wave guideshave been produced by doping, or semiconductor substrates, for examplefrom silicon, may be used as substrate on which the wave guides areproduced via an oxide layer, for example silicon oxide, by glass or anorganic matter. Instead of the thermo-optical phase modulators mentionedin connection with the embodiments, it also is possible to useelectro-optical phase modulators, if the material of the wave guide tobe influenced shows electro-optical activity, i.e. shows anelectro-optical effect sufficient for the phase modulation, when anelectrical field is applied to the wave guide. In these cases, theelectrical field is generated by to electrodes between which the waveguide extends and which are connected to a voltage source. Phasemodulators of that kind are for example suitable for use with theabove-mentioned wave guides which have been generated by titaniumdiffusion or proton exchange in lithium niobate crystals or by dotingIII-V semi-conductors. Finally, in particular in connection with thefirst embodiment, light guide fibers have been described as wave guidesbetween the interferometer and the photodetectors. Is is, however, alsopossible to use wave guides like those described in connection with theinterferometer chip.

We claim:
 1. An interfermometer comprising:a light source; a firstwaveguide formed on a substrate, the first waveguide splitting at afirst location into a first coupler waveguide and a second couplerwaveguide and combining at a second location to form a measuring arm; aphase modulator coupled to the measuring arm portion of the firstwaveguide; a second waveguide formed on the substrate, a portion of thesecond waveguide positioned adjacent to the first coupler waveguide, thesecond waveguide forming a first reference arm; a third waveguide formedon the substrate, a portion of the third waveguide positioned adjacentto the second coupler waveguide, the third waveguide forming a secondreference arm, the first reference arm having an optical path lengthdifferent from the path length of the second reference arm; means forguiding a beam of light from the light source to the first waveguide,the light being coupled into the second and third waveguides by thefirst and second coupler waveguides wherein the light traveling throughthe first and second reference arms of the second and third waveguidesis reflected back through the second and third waveguides and broughtinto interference with light in the measuring arm reflected back from anobject; a plurality of detectors coupled to the second and thirdwaveguides for detecting the interference signals; and an evaluationcircuit coupled to the plurality of detectors for detecting a change inoptical path length and the direction of change, wherein the first,second and third waveguides are formed in a substrate by ion exchange.2. An interferometer comprising:a light source; a first waveguide formedon a substrate, the first waveguide splitting at a first location into afirst coupler waveguide and a second coupler waveguide and combining ata second location to form a measuring arm; a phase modulator coupled tothe measuring arm portion of the first waveguide; a second waveguideformed on the substrate, a portion of the second waveguide positionedadjacent to the first coupler waveguide, the second waveguide forming afirst reference arm; a third waveguide formed on the substrate, aportion of the third waveguide positioned adjacent to the second couplerwaveguide, the third waveguide forming a second reference arm, the firstreference arm having an optical path length different from the pathlength of the second reference arm; means for guiding a beam of lightfrom the light source to the first waveguide, the light being coupledinto the second and third waveguides by the first and second couplerwaveguides wherein the light traveling through the first and secondreference arms of the second and third waveguides is reflected backthrough the second and third waveguides and brought into interferencewith light in the measuring arm reflected back from an object; aplurality of detectors coupled to the second and third waveguides fordetecting the interference signals; an evaluation circuit coupled to theplurality of detectors for detecting a change in optical path length andthe direction of change; a plurality of amplifiers coupled to theplurality of detectors, a plurality of pulse generators coupled to theamplifiers, a driving unit coupled to the phase modulator, a directiondiscriminator coupled to the pulse generators, a counter coupled to thedirection discriminator and a first display unit coupled to the counter;a function generator coupled to the driving unit for driving the phasemodulator, a phase meter coupled to the function generator and to one ofthe detectors and a second display unit coupled to the phase meter; anda memory coupled to the output of the function generator, the memoryalso being coupled to an output of the counter and an input of the firstdisplay.
 3. An interferometer according to claim 2 wherein the functiongenerator generates saw-tooth signals to control the phase modulator andthe phase between the saw-tooth signal and the signal from the onedetector is measured in the phase meter and displayed on the seconddisplay.
 4. An interferometer according to claim 2 wherein the functiongenerator generates a stepped digital modulation signal having a 2 Piphase rotation within one period.
 5. An interferometer according toclaim 2 wherein the function generator generates a digital signal whichruns in a meander shape about a saw-tooth curve.
 6. An interferometeraccording to claim 2 wherein the function generator generates a signalhaving a frequency of one (1) kiloHertz.
 7. A method for determining thechange in optical path length and the direction in change using aninterferometer, comprising the steps of:guiding light from a lightsource in a measuring arm, a first reference arm and a second referencearm, the first and second reference arms having different optical pathlengths; interfering the light guided in the measuring arm with lightguided in the first and second references arms; modulating the lightguided in the measuring arm by providing a phase modulator at themeasuring arm; detecting the interference between the light guided inthe measuring arm and the light guided in the first and second referencearms; and applying the detected signals to an evaluation unit, whereinthe step of modulating the light beam in the measuring arm includes oneof either adjusting a potentiometer and by a programmable controller.